ar71xx: increase kernel partition size for TP-Link RE450 v1

This increases kernel partition size and fixes rootfs (file-system)
partition size on TP-Link RE450 v1. Also, while we are at it, switch
from statically defined kernel and rootfs partitions in kernel cmdline
to "tplink-fw" mtd splitter.

Fixes: FS#1072.

Signed-off-by: Piotr Dymacz <pepe2k@gmail.com>

target/linux/ar71xx/image/tp-link.mk

@@ -270,7 +270,7 @@ TARGET_DEVICES += eap120-v1
 define Device/re450-v1
   DEVICE_TITLE := TP-LINK RE450 v1
   DEVICE_PACKAGES := kmod-ath10k ath10k-firmware-qca988x
-  MTDPARTS := spi0.0:128k(u-boot)ro,1344k(kernel),4672k(rootfs),64k(partition-table)ro,64k(product-info)ro,1856k(config)ro,64k(art)ro,6016k@0x20000(firmware)
+  MTDPARTS := spi0.0:128k(u-boot)ro,6016k(firmware),64k(partition-table)ro,64k(product-info)ro,1856k(config)ro,64k(art)ro
   IMAGE_SIZE := 7936k
   BOARDNAME := RE450
   TPLINK_BOARD_ID := RE450

tools/firmware-utils/src/tplink-safeloader.c

@@ -852,8 +852,8 @@ static struct device_info boards[] = {
 		*/
 		.partitions = {
 			{"fs-uboot", 0x00000, 0x20000},
-			{"os-image", 0x20000, 0x150000},
-			{"file-system", 0x170000, 0x4a0000},
+			{"os-image", 0x20000, 0x180000},
+			{"file-system", 0x1a0000, 0x460000},
 			{"partition-table", 0x600000, 0x02000},
 			{"default-mac", 0x610000, 0x00020},
 			{"pin", 0x610100, 0x00020},